The Boy Scouts of America have announced plans to admit girls into the century-old youth club, allowing them to achieve the same high rank as boys.

The historic decision was agreed upon unanimously by the group's board of directors on Wednesday, triggering a fierce backlash online.

Under the new rules, girls can join the Cub Scouts at a young age, and eventually graduate as Eagle Scouts.

The Girl Scouts of the USA are among critics of the move.

Boy Scouts of America (BSA) chief executive Michael Surbaugh said in a statement: "We believe it is critical to evolve how our programs meet the needs of families interested in positive and lifelong experiences for their children."

"Families today are busier and more diverse than ever," the BSA statement continued. "Most are dual-earners and there are more single-parent households than ever before, making convenient programs that serve the whole family more appealing."

The decision was announced on the United Nations' International Day of the Girl, which has been observed since 2012 and is intended to highlight issues facing women and girls.

The move was swiftly disparaged by some social media users, including the eldest son of US President Donald Trump.

Starting in 2019, girls will be allowed to become Eagle Scouts, a prestigious honour - if they complete a volunteer service project.

In 2018, parents will be able to enlist their children in Cub Scout programs.

Each chapter will be able to determine if they want to remain exclusive to boys, create all-female groups or establish new mixed-sex "packs".

Over the summer, Girl Scout president Kathy Hopinkah Hannah wrote an open letter to Boy Scouts officials accusing them of a "covert campaign" to recruit girls into the organization, which she said is confronting "well-documented" declining membership.

"I formally request that your organization stay focused on serving the 90 percent of American boys not currently participating in Boy Scouts... and not consider expanding to recruit girls," Hannah wrote to Boy Scouts president Randall Stephenson in August.

In January, the Boy Scouts announced plans to welcome a transgender boy into the group.

The BSA reports having about 2.3m members in the US, down about a third since 2000. (Source: BBC News)